The present invention relates to a method and an apparatus for generating energy. In particular, it relates to a method and an apparatus for generating energy in which an air separation unit sends a nitrogen-enriched gas stream upstream of an expansion machine which generates energy by expanding the combustion gases.
It also relates to methods and apparatus for separating air adapted to be integrated into an energy generating method of this type.
Various schemes have been proposed to integrate gas turbines and units for separating air by cryogenic distillation, in particular within the context of IGCCs and units for separating air by cryogenic distillation operating at high pressure.
Typically, as described in U.S. Pat. No. 4,224,045, air is taken from the air compressor of the gas turbine to supply, at least partly, the air separation unit which in return sends nitrogen either into the fuel intended for the combustion chamber or upstream of the expansion machine of the turbine.
In U.S. Pat. No. 4,382,366, which is the closest prior art, all the air compressed in a compressor coupled to a gas turbine is sent to a single column. The combustion chamber is supplied with fuel and impure nitrogen coming from the reversible exchangers of the air separation unit.
EP-A-0465193 describes a method in which the compressor coupled to the expansion machine sends no air to the air separation unit.
One aim of the present invention is to simplify the design of the combustion chamber.
Another aim of the invention is to reduce the production of NOxs by the gas turbine.
According to one objective of the invention, provision is made for a method of generating energy using an energy generation unit comprising the steps of:
i) compressing air in a compressor;
ii) sending at least part of the air compressed in the compressor to an air separation unit in order to produce at least one oxygen-enriched fluid and at least one nitrogen-enriched gas also containing oxygen;
iii) sending fuel and at least part of the nitrogen-enriched gas to a combustion chamber in order to produce combustion gases, the air compressed in the compressor not being sent to the combustion chamber; and
iv) expanding the combustion gases in an expansion machine coupled to the compressor in order to recover energy;
characterized in that the nitrogen-enriched gas is compressed to a pressure between 8 and 30 bar before being sent to the combustion chamber.
Thus, since all the air from the compressor of the gas turbine is sent to the air separation unit, the combustion chamber is simplified.
Combustion with the oxygen contained in a gas stream of the nitrogen-enriched air coming from an air separation unit makes it possible to produce very little NOx.
According to other optional aspects of the invention:
the air from the compressor is sent to the air separation unit;
part of the air from the compressor is sent to the air separation unit and the rest of the air compressed in the compressor serves to cool at least one element of the unit other than the combustion chamber;
the air sent to the air separation unit comes from the compressor;
part of the air sent to the air separation unit comes from a makeup compressor or a source of pressurized air;
the air from a makeup compressor is sent to the combustion chamber;
the air from the makeup compressor is mixed with at least part of the nitrogen-enriched gas before being sent to the combustion chamber;
at least part of the oxygen-enriched gas is sent in order to gasify a fuel containing carbon so as to generate a fuel stream;
the only gas sent to the combustion chamber apart from the fuel is the nitrogen-enriched gas;
the nitrogen-enriched gas contains at least 5 mol % and at most 18 mol % of oxygen;
another gas stream containing oxygen other than the fuel and the nitrogen-enriched gas is sent to the combustion chamber;
the other gas stream comprises from 2 to 100 mol % of oxygen;
the nitrogen-enriched gas contains less than 18 mol % of oxygen;
the nitrogen-enriched gas contains less than 5 mol % of oxygen;
the air is compressed by the compressor to between 8 and 20 bar.
According to another objective of the invention, provision is made for an energy generating apparatus comprising:
i) a compressor;
ii) an expansion machine coupled to the compressor;
iii) a combustion chamber;
iv) an air separation unit;
v) means for sending the air from the compressor to the air separation unit;
vi) means for sending a nitrogen-enriched gas containing oxygen from the air separation unit to the combustion chamber and no means for sending air from the compressor to the combustion chamber;
characterized in that it comprises means for compressing the nitrogen-enriched gas before sending it to the combustion chamber.
According to other optional aspects, provision is made for:
a makeup compressor for sending air to the air separation unit;
a gasifier, means for sending an oxygen-enriched gas from the air separation unit to the gasifier and means for sending fuel from the gasifier to the combustion chamber.
According to another objective of the invention, provision is made for a method of separating air in an unit comprising at least three columns in which compressed and purified air is sent to a first column, a nitrogen-enriched stream and an oxygen-enriched liquid are extracted from the first column, the oxygen-enriched stream is sent to a second column, a stream is removed from the head of the second column, at least part of the liquid in the bottom of the second column is sent to a third column and a second oxygen-enriched stream and a second nitrogen-enriched stream are withdrawn from the third column, the third column operating at a lower pressure than the second column and being thermally connected thereto by means of a reboiler/condenser, characterized in that compressed and purified air is sent to at least some trays above the bottom of the first column and a bottom reboiler of the first column is heated by another stream.
According to other optional aspects:
means for sending the liquefied air in the bottom boiler from the first column to the second and/or to the third column;
the first column operates substantially at the same pressure as the second column;
means for compressing the nitrogen-enriched gas before sending it to the combustion chamber.
According to another objective of the invention, provision is made for an air separation apparatus comprising at least three columns, means for sending air to a first column, means for sending an oxygen-enriched stream from the first column to the second column, a reboiler/condenser thermally connecting the head of the second column and the bottom of the third column, means for extracting a stream from the head of the second column, means for sending at least part of the liquid at the bottom of the second column to a third column and means for withdrawing a second oxygen-enriched stream and a second nitrogen-enriched stream from the third column, characterized in that it comprises means for sending compressed and purified air to the first column above at least one theoretical tray thereof, a reboiler at the bottom of the first column and means for sending a heating gas to the bottom reboiler.
According to another optional aspect, provision is made for:
means for withdrawing a stream from the head of the second column.
So as to optimize the operation of the combustion chamber, the oxidizer may be a mixture of waste nitrogen from an ASU (air separation unit) and makeup air so as to control the oxygen content.